Discovery of ligands that modulate receptor function by binding to allosteric sites on biological targets has emerged as a promising new approach for finding drugs possessing significant therapeutic advantages over drugs that act as orthosteric ligands. Such allosteric modulators are currently being explored in-depth in the field of G-protein-coupled receptors. Among the membrane bound targets in the central nervous system, the biogenic amine transporters in general, and the dopamine (DA) transporter (DAT) in particular, play a key role in addiction to stimulant drugs of abuse and in a number of psychiatric illnesses. Although ligands that are allosteric modulators of DAT have been of considerable interest, discovery of such allosteric modulators of DAT has hitherto remained elusive. Our recent discovery of allosteric modulatory effects among a series of drug-like small molecules represents a significant breakthrough in the search for allosteric ligands of DAT. Studies with these ligands have shown that they partially inhibit DAT binding, [3H]DA uptake, and D- amphetamine induced DA release. Moreover, evaluation of a few analogues of these compounds has revealed emerging structure-activity relationship (SAR) trends among the congeners, thus setting the stage for further research on allosteric modulators of DAT function. Our goals are to pursue these initial leads to identify ligands with improved potency and endowed with favorable physicochemical properties for their use as in vitro and in vivo tools to study the biological consequences of allosteric modulation of DAT function in health and disease. Our specific aims in the R21 phase include iterative design, procurement and synthesis and evaluation of focused libraries of compounds to rapidly identify a set of ligands suitable for in vivo studies in the R33 phase. SAR analysis and QSAR model development will be utilized to pursue ligand-structure based approaches for the rational design of new focused libraries. Selected promising ligands will be evaluated against serotonin transporter (SERT) and norepinephrine transporter (NET) to ascertain their transporter target selectivity. Our expectations are that by the end of the two year R21 phase we will have identified a select set of compounds with sufficient potency, efficacy and selectivity for further studies to be performed in the next phase. The specific aims to be pursued in the R33 phase include (a) profiling the compounds against a panel of targets, (b) determination of systemic bioavailability and brain penetration properties, and (c) design and synthesis of radiolabels to serve as tools for characterizing the novel binding site. Envisioned in vivo studies with candidate molecules include (i) microdialysis experiments in rats (ii) locomotor studies in rats and (iii) determination of the effects of the compounds alone or in combination with cocaine in (1) discrimination assays in rats and monkeys, (2) intracranial self-stimulation assays in rats, and (3) self-administration studies in monkeys. These studies should reveal the potential of the allosteric ligands of DAT as treatment agents for substance abuse and psychiatric disorders.